1. Field of the Invention
The present invention relates generally to an exposure control system for camera and other optical instruments, and particularly to a motion detecting device for detecting the relative motion rate or the velocity between an object and the instrument in the exposure control system by which the exposure time, i.e. the shutter speed of the instrument is controlled in response to the relative motion rate or velocity.
2. Description of the Prior Art
Generally, it is well known that the shutter speed of the camera is a very important factor in making a exposure decision. The shutter speed has a close connection with the motion of the object or the camera so that for an object in a quick motion it is necessary to select a quick shutter speed. So far the shutter time has been selected in accordance with the visual measurement of the photographer, which is not perfect. Especially in the case of a camera with an automatic exposure control device, the shutter time is selected independently of the intention of the photographer or the motion of the object so that a good picture cannot always be obtained, and in order to avoid problem, it is necessary to manually alter or modify the preset value, which is problematic.
In order to solve such a problem, an automatic exposure control system for a camera so designed as to detect the relative motion rate or velocity between the object to be photographed and the camera in order to automatically control the exposure time, i.e. the shutter speed in accordance with the detected relative motion rate or velocity has already been proposed by the U.S. Pat. No. 3,399,307 (patented on Aug. 27, 1968 for the invention entitled "Motion Sensing Exposure System for Optical Instruments"). The motion sensing exposure control system proposed by the U.S. Pat. No. 3,399,307 is the one so designed as to obtain a D.C. signal of a level corresponding to the relative motion rate or velocity between the object and the camera. It operates when the object moves with reference to the camera, the camera moves with reference to the object or the camera and the object move with reference to each other. The D.C. signal generated by the movement is used to control the shutter speed and the diaphragm aperture (f-stop).
When the exposure control system of this proposal is employed in front of a photo-electric cell, such as photo resistor arranged to receive the object image from the optical system, a light chopping element, such as grating composed of alternate transparent and opaque areas, is used to sense or detect the motion. A monostable multivibrator, triggered by the output of the photo-electric cell and a time integrator for integrating the output of the monostable multivibrator in accordance with the lapse of time is also employed. When a relative motion occurs between the object and the camera, an A.C. output with a frequency corresponding to the then relative motion rate or speed is then obtained and this output triggers the monostable multivibrator, the output of the monostable multivibrator is then integrated with a time integrator related to the time. A D.C. signal whose level corresponds in frequency to the A.C. output of the photo-electric cell is then obtained, and the level of this D.C. signal corresponds to the relative motion rate or velocity between the object and the camera.
But the use of a grating in front of the photo-electric cell can give rise to misdetection due to periodic or random fluctuation of the object brightness. For example, if the object to be photographed is illuminated with a fluorescent lamp, the object brightness fluctuates due to the illumination with the fluorescent lamp. In this case the photo-electric cell produces an A.C. signal with a frequency corresponding to the fluctuation frequency of the object brightness. Even if there is no relative motion between the object and the camera in this case, the photo-electric cell produces an A.C. signal so that the misdetection takes place due to the flourescent fluctuation of the object brightness. Of course, the above is an extreme example. However, as can be understood from this example, it is clear that misdetection takes place due to nonmotion imposed fluctuation, if any, of the object brightness.
Another problem tends to arise when employing the teachings of U.S. Pat. No. 3,399,307; if the motion detecting device uses photo-resistors as the photo-electric cell. Here the change of the total resistance value of the photo-resistors equals the sum of the changes of the individual resistance values in each image receiving portion. When the object image comprises a comparatively complicated pattern of brightness and darkness, the changes in the resistance values in the image receiving portion cancell each other when the object is in motion. Should this occur, no change in the measured resistance value or only a small change, can take place, thus tending to destroy the ability to detect the relative motion rate or velocity between the object and the camera. Especially, where the object to be photographed has a very complicated pattern, thus making it still more difficult to detect motion using the apparatus described in U.S. Pat. No. 3,399,307.
As explained above, the motion detecting device proposed by U.S. Pat. No. 3,399,307 is fraught with problems needing solution.